A few details that would be really great for challenges that I haven’t seen suggested (inspired by a Reddit thread)

• Ability to filter your challenge prompts to ‘only with books added’, ‘only without books added’ ‘only complete’, ‘only incomplete’ etc. as you can
  from the ‘Add Book to Reading Challenge’ prompt

• Ability to search prompts

• Ability to filter the books already added to a prompt by ‘on TBR’ or ‘Read’

• Ability to filter books already added to prompt by standard filters (A-Z etc.) and maybe by rating

• Challenge page not resetting to top of the list when you leave the tab

• Filtered search on personal TBR and Read lists not resetting when you leave that tab, so that you can easily switch back and forth between your lists to find matching books.